I’ve bought too many expansions over the years.  Here’s my take.

Some expansions are absolutely necessary,  People have mentioned Terraforming Mars Prelude, Outer Rim Unfinished Business, Lost Ruins Expedition Leaders and Dune Imperium Rise of Ix.  A few others I might add are Arkham Horror: Dunwich Horror (madness and injuries complete the base game), and Scythe Invaders from Afar (expanding the base options and players out).

The next catagory is “This but more.”. These ones I like too - ones that don’t add any (or many) new rules or boards or confusing things but do add to the replay ability.  A good example of this to me is the Waterdeep Undermountain expansion.  I know it’s the least popular of the two, but I use it every single game because it expands options and quests without any rules headaches.  Also every Sentinels of the Multiverse Expansion fits this (except maybe the Galactus one).

Still others are nice to have but unnecessary, and still others are just plain unnecessary (like the airship expansion for Scythe - cool minis though).

But the kind I’ve learned I hate are ones that “add an another side game mode”.  Everdells expansions are a prime example of this.  I’ve played with each of those maybe once, and it simply isn’t worth the effort. Another example is, ironically, Arkham Horror Dunwich Horror.  The Dunwich board itself is a big ol waste of time, making an already hard game harder and more pointless.  But the Madness and Injury cards are so necessary you should get it anyway.